The present invention relates generally to industrial curtains used as environmental closures for openings through which traffic can still pass. The curtains generally comprise a plurality of strips suspended contiguously to each other from a hanger fixed adjacent to a top margin of the opening, each strip consisting essentially of a length of flexible material terminating adjacent to a lower margin of the opening. The present invention relates particularly to an improved configuration for the material forming the strips of such industrial curtains so that vision through the curtain is improved in situations involving significant differences in environmental temperature on opposite sides of the curtain, and to a structure for suspending the strips.
Goods are often required to be transported from one area of a manufacturing or storage facility to another, where one of the areas is heated or ambient while the adjacent area is air-conditioned or even refrigerated. Where the traffic is only occasional, conventional doors can be employed to close any doorway between the two areas. Where the traffic is considerable, the use of conventional doors gives way to suspended flexible screens or curtains that inhibit the wholesale transfer of heated or refrigerated air from one area to the other yet still permit goods-transporting vehicles and personnel to pass through with little effort. Early screens were sometimes made of rubber as shown, for example, in U.S. Pat. No. 2,122,532. For safety reasons, it is desirable that the curtain be sufficiently transparent that one operating a transporting vehicle be able to see any hazard or obstruction that might exist on an opposite side of a screen before proceeding through. Persons on the opposite side of a screen also desire to be able to see oncoming transport vehicles so appropriate evasive action can be taken. Thus, plastic materials, which were more or less transparent, such as polyvinyl chloride and polyethylene, were adopted as the preferred materials for such screens as shown, for example, in U.S. Pat. Nos. 4,095,642; 4,165,778; 4,232,725; 4,367,781; and 4,607,678.
In situations involving significant differences in environmental temperature on opposite sides of the curtain, the large temperature difference often causes the humidity in the air, generally on the warmer side of the curtain, to condense on the surface of the curtain. If the temperature on the cooler side of the curtain is sufficiently low, the accumulating condensation turns to frost or even ice. The resulting condensation, whether or not frozen, contributes to a significant loss in visibility through the screen that results in a corresponding increase in hazard for transport vehicle operators and others alike. The condensation is often observed to drip off the lower end of the curtain and pool on the floor in the doorway. The pool of water, or ice if the area is sufficiently cold, constitutes an added hazard that would be desirable to avoid, if possible.
In an effort to diminish the likelihood of condensation or frost development, several designs for curtains and associated structures have been suggested by the prior art. The perimeter of a doorway may be kept free from frost by embedding a heater in the form of an electrical resistance unit in a doorway edge as disclosed in U.S. Pat. No. 4,855,567. Attempts at reducing the frost or condensation on a door or curtain have included the use of radiant heat as shown in U.S. Pat. No. 4,950,869 and warmed air blowers as shown in U.S. Pat. Nos. 4,288,992; 4,400,046; and 5,203,175. None of the systems have proven to be entirely satisfactory. The electrical resistance units may perform satisfactorily for a frame surrounding a doorway, but do not measurably affect the presence of frost or condensation on a door or curtain in the same doorway. The radiant heaters are observed to deliver heat unevenly, with the portion of the door or curtain closest to the heater receiving too much heat, and the portion farthest away receiving too little heat. Air convection currents generally prevent warmed air blowers, positioned at either the top or sides of a doorway, from delivering a sufficient amount of air to the bottom-central portion of a door or curtain to achieve the desired effect without resorting to very high-power blower units.
There is therefore a continuing need for alternative mechanisms for achieving a frost- and condensation-free industrial curtain while consuming as little power as possible. Preferably, the alternative mechanisms would also minimize or prevent any pooling of water at the base of the curtain, thereby avoiding that added hazard.
In accordance with the present invention, a substantially transparent thermal barrier is positioned for use in an opening between a refrigerated area and an adjacent area having a significantly higher temperature. The thermal barrier is suspended in the opening by its upper margin, where it is connected to a header. The barrier comprises a plurality of strips positioned adjacent to each other so as to present a substantially continuous barrier between the two areas on either side of the opening. Each of the strips has an upper and a lower end, the upper end being supported by the header. Each strip can include at least two sheets of substantially transparent material fixed to each other so as to define a plurality of channels extending from the upper end to the lower end of each strip, with at least the upper ends of the channels being open. Alternatively, each strip can be viewed to comprise a plurality of vertical tubes or conduits connected laterally to each other, with at least the upper ends of each tube being open. The header includes means for directing a flow of air into the open upper ends and then downward through the plurality of channels, conduits or tubes of each of the strips. The cross-sectional shape of the tubes, conduits or channels can be varied, and can be seen to form ducts for conducting the air downward from the header. The ducts generally will have open lower ends permitting escape of the flow of air, but can have closed lower ends that include lateral openings to adjacent ducts for conducting the air upward toward the header.
The air to be introduced into the open upper ends of the ducts should contain as little moisture as possible so that no condensation takes place within the ducts. For example, air is preferably taken from the colder of the two adjacent areas separated by the curtain since the air in that area, being colder, is likely to have less retained moisture. Prior to being introduced into the ducts in the curtain, the air is preferably heated to a temperature that is at least about midway between the temperatures of the two areas separated by the curtain. Air that has a temperature that is at least about midway between the temperatures of the two areas separated by the curtain is hereby defined as warmed air. To achieve this limited heating, thermostatic sensors can be positioned in each of the areas separated by the curtain to sense the temperature in each area. The thermostatic sensors can be connected to the heater by an appropriate control that prevents overheating of the air, which would be a waste of energy. As the warmed air proceeds downward through the length of the ducts, the air does not significantly change its temperature. As a result, the warmed air flowing downward through the ducts is delivered substantially at floor level.
If the ducts have open lower ends, the warmed air flowing downward through the ducts is delivered immediately below the curtain and generally uniformly across the width of the opening in which the curtain is positioned. The warmed air in the ducts generally maintains the surface of the curtain confronting the warmer area at a higher temperature that would be the case without the downward flow of air, thus inhibiting the formation of condensation or frost on the curtain surface. Additionally, the natural convection currents now cause the warmed air to flow upward on either side of the curtain, this air flow forming a moving blanket that additionally inhibits the formation of condensation or frost on the curtain surface. The same effect could be achieved by introducing air from which substantially all the moisture had been removed, herein defined as dried air. The warmed air and dried air are referred to herein as conditioned air.
It will be appreciated that the downward flow of conditioned air can be captured by means enabling the return of this air to the plenum. This use of return air has the advantage of lowering the amount of energy used in conditioning the air since the air temperature will be approximately the average temperature of the two areas separated by the curtain. This return of the air to the plenum can be achieved by providing an adjacent set of ducts, connected to the lower ends of the ducts containing the downward flowing air, for returning the air upward toward the plenum. The upper ends of the return ducts can merely open into one of the adjacent areas, or can be directed outside the structure entirely. Preferably, the upper ends of the return ducts are connected to an intake portion of the header so that the conditioned air is re-circulated in a manner reducing the overall energy demand of the system. The return ducts can be positioned in a variety of patterns and can form an additional layer of each strip forming the curtain.
The return ducts can also be separate from the strips forming the curtain. For example, the return ducts can constitute an open channel in the floor immediately below the lower end of a curtain having open lower ended downward flowing ducts. The channel can then be connected to ducting on one or both sides of the opening in which the curtain is installed, the ducting leading to the plenum where the returned air can be heated and dried as necessary before reintroduction into the header coupled to the upper end of the curtain.
The warming of the air to be introduced into the ducts can be achieved using any convenient heating element and an appropriate source of power. A particularly advantageous mechanism for warming the flow of air is a large area heat exchanger that merely draws heat from the air in the warmer area, or perhaps from air exterior to the structure as a whole. Such an air warming mechanism does not use energy in the warming process itself, although it would still require the use of some energy to move the air through the heat exchanger and down the ducts of the curtain strips. Alternatively, with the aid of a heat pump, it is conceivable that the heat exchanger would be positioned in the cooler area and thereby aid in maintaining the temperature difference between the two areas. Further, the heat pump can be used to heat or dry the air, or both so as to arrive at the desired level of conditioning.
The conditioned air can be sub-divided into two or more flows of air where there are more than two sheets employed to make curtain strips having multiple layers of ducts rather than just a single layer or set of ducts. Where the air is sub-divided into two or more flows, the temperatures of the flows can be varied from each other by providing additional air warmers for the flow adjacent the area having the higher ambient temperature.
One feature of the present invention is a header that can be affixed to or form a part of the top of an opening in a wall between a cooled or refrigerated area and an adjacent area having a significantly higher temperature. The header is intended to support an industrial curtain of the type described herein and includes means for directing a flow of air into the upper end of the plurality of ducts of each of the strips forming such an industrial curtain. The header includes a plenum that preferably draws in air from the cooler area and warms the air in a heater or heat exchanger. The warming of the previously cooled air has the advantage of drying the air thus increasing the capacity of the air to evaporate any moisture that it might encounter.
Another feature of the present invention is the industrial curtain formed from a plurality of strips, each strip comprising at least two sheets of material fixed to each other so as to define a plurality of open-ended ducts extending from the top to the bottom of the curtain. The open ended ducts are intended to be connected to the plenum of the header so that the conditioned air is internally conducted to the bottom of the industrial curtain prior to any interaction with the surrounding air or with any frost or condensation that might have previously formed. This structure has the advantage of locating a flow of conditioned air at the bottom of the industrial curtain so that it can flow by convection up either side of the curtain to remove any condensation present and prevent any further formation of condensation or frost on the curtain.
Other features and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following discussion of preferred embodiments of the invention that are illustrated in the accompanying drawings.